Re: [PATCH v5 1/2] mm/madvise: optimize lazyfreeing with mTHP in madvise_free

[David Hildenbrand]

On 11.04.24 13:11, Ryan Roberts wrote:
> On 08/04/2024 05:24, Lance Yang wrote:
> > This patch optimizes lazyfreeing with PTE-mapped mTHP[1]
> > (Inspired by David Hildenbrand[2]). We aim to avoid unnecessary folio
> > splitting if the large folio is fully mapped within the target range.
> >
> > If a large folio is locked or shared, or if we fail to split it, we just
> > leave it in place and advance to the next PTE in the range. But note that
> > the behavior is changed; previously, any failure of this sort would cause
> > the entire operation to give up. As large folios become more common,
> > sticking to the old way could result in wasted opportunities.
> >
> > On an Intel I5 CPU, lazyfreeing a 1GiB VMA backed by PTE-mapped folios of
> > the same size results in the following runtimes for madvise(MADV_FREE) in
> > seconds (shorter is better):
> >
> > Folio Size |   Old    |   New    | Change
> > ------------------------------------------
> >        4KiB | 0.590251 | 0.590259 |    0%
> >       16KiB | 2.990447 | 0.185655 |  -94%
> >       32KiB | 2.547831 | 0.104870 |  -95%
> >       64KiB | 2.457796 | 0.052812 |  -97%
> >      128KiB | 2.281034 | 0.032777 |  -99%
> >      256KiB | 2.230387 | 0.017496 |  -99%
> >      512KiB | 2.189106 | 0.010781 |  -99%
> >     1024KiB | 2.183949 | 0.007753 |  -99%
> >     2048KiB | 0.002799 | 0.002804 |    0%
> >
> > [1] https://lkml.kernel.org/r/20231207161211.2374093-5-ryan.roberts@xxxxxxx
> > [2] https://lore.kernel.org/linux-mm/20240214204435.167852-1-david@xxxxxxxxxx
> >
> > Signed-off-by: Lance Yang <ioworker0@xxxxxxxxx>
> > ---
> >   include/linux/pgtable.h |  34 +++++++++
> >   mm/internal.h           |  12 +++-
> >   mm/madvise.c            | 149 ++++++++++++++++++++++------------------
> >   mm/memory.c             |   4 +-
> >   4 files changed, 129 insertions(+), 70 deletions(-)
> >
> > diff --git a/include/linux/pgtable.h b/include/linux/pgtable.h
> > index 0f4b2faa1d71..4dd442787420 100644
> > --- a/include/linux/pgtable.h
> > +++ b/include/linux/pgtable.h
> > @@ -489,6 +489,40 @@ static inline pte_t ptep_get_and_clear(struct mm_struct *mm,
> >   }
> >   #endif
> >   
> > +#ifndef mkold_clean_ptes
> > +/**
> > + * mkold_clean_ptes - Mark PTEs that map consecutive pages of the same folio
> > + *		as old and clean.
> > + * @mm: Address space the pages are mapped into.
> > + * @addr: Address the first page is mapped at.
> > + * @ptep: Page table pointer for the first entry.
> > + * @nr: Number of entries to mark old and clean.
> > + *
> > + * May be overridden by the architecture; otherwise, implemented by
> > + * get_and_clear/modify/set for each pte in the range.
> > + *
> > + * Note that PTE bits in the PTE range besides the PFN can differ. For example,
> > + * some PTEs might be write-protected.
> > + *
> > + * Context: The caller holds the page table lock.  The PTEs map consecutive
> > + * pages that belong to the same folio.  The PTEs are all in the same PMD.
> > + */
> > +static inline void mkold_clean_ptes(struct mm_struct *mm, unsigned long addr,
> > +				    pte_t *ptep, unsigned int nr)
>
> Just thinking out loud, I wonder if it would be cleaner to convert mkold_ptes()
> (which I added as part of swap-out) to something like:
>
> clear_young_dirty_ptes(struct mm_struct *mm, unsigned long addr,
> 		       pte_t *ptep, unsigned int nr,
> 		       bool clear_young, bool clear_dirty);
>
> Then we can use the same function for both use cases and also have the ability
> to only clear dirty in future if we ever need it. The other advantage is that we
> only need to plumb a single function down the arm64 arch code. As it currently
> stands, those 2 functions would be duplicating most of their code.

Yes. Maybe better use proper __bitwise flags, the compiler should be 
smart enough to optimize either way.

--
Cheers,

David / dhildenb